The instant invention relates to control circuits for vehicle passenger safety restraint systems, such as air bags, comprising a firing path which includes at least two acceleration sensors whose acceleration-responsive switches are connected in parallel with one another for purposes of redundancy.
Known air bag passenger restraint systems employ a control circuit wherein a power supply applies a voltage across a firing path which includes in series an explosive squib and at least two acceleration sensors having normallyopen acceleration-responsive switches therein. The switch of each sensor is shunted by a resistor having a nominal resistance substantially greater than the internal resistance of the squib. Thus, a small current nominally flows through the firing path while the switches of the sensors remain in their normally-open positions. The closure of the sensors's switches in response to a collision or marked vehicle deceleration causes a significant rise in the current flowing through the squib, thereby "firing" the squib and triggering deployment of the air bag. See, e.g., U.S. Pat. No. 4,695,075, issued Sept. 22, 1987 to Kamiji et al.
In U.S. Pat. No. 4,851,705 issued July 25, 1989, Musser et al. teach an air bag firing circuit comprising two parallel crash sensors and two parallel safing sensors connected to an explosive squib so as to trigger the deployment of an air bag upon the simultaneous closure of one or more crash sensors and one or more safing sensors. The firing circuit thus provides redundant firing paths to permit continued operation of the firing circuit notwithstanding the failure a crash sensor and/or a safing sensor in its "open" position, or in the event that one such redundant sensor is disconnected from the circuit as a result of a crash condition. The use of such redundant crash and safing sensors further permits the positioning of such sensors in different locations about the vehicle to increase the variety of vehicle acceleration conditions sensed thereby, whereby the vehicle passengers are afforded still greater levels of protection. Musser et al. further teaches the use of a pair of diode bridges to obtain full firing circuit diagnosability, whereby the malfunction of any given circuit component, including the crash and safing sensors, is readily detected.
Unfortunately, upon the detection of a failure of a redundant sensor in the closed position, or with a "propensity to close", the viability of the firing circuit is severely impaired by the resulting short-circuiting of the stilloperable sensor located in parallel with the failing sensor.
Under the prior art, if the switch of either sensor fails in its closed position, or with a propensity to close, the prior art teaches the disabling of the entire control circuit to prevent the unintentional or premature triggering of the passenger restraint, once again placing the passengers at risk. See, e.g., U.S. Pat. No. 3,889,232, issued June 10, 1975 to Bell, wherein the control circuit shuts down when one sensor closes without the corresponding closing of the other sensor.
Alternatively, in our U.S. Pat. No. 4,958,851 issued Sept. 25, 1990, we teach an air bag firing circuit whose firing path comprises at least two acceleration sensors connected in series with an explosive squib. The firing circuit further includes means for functionally removing a malfunctioning sensor from the firing path by closing or shunting the malfunctioning sensor, thereby providing continued protection of the vehicle passengers under the control of the remaining, still-operable sensor(s). However, such closure or shunting of the failing sensor necessarily removes any operable, redundant sensor, i.e., one connected in parallel with the failing sensor, from the firing path, with an attendant reduction in the protection afforded vehicle passengers by the safety restraint.